1. Technical Field
The present invention relates to drying apparatuses, and more particularly, to a vacuum drying apparatus that uses technologies of condensation and vacuum to dry surfaces of granulous or powdered substances.
2. Description of Related Art
For an injection-molding device, it is preferred that plastic material granules processed therein are dry enough to produce products of satisfying quality.
A conventional drying apparatus comprises a hopper, a heater and a blower. When material granules are loaded into the hopper, the heater heats an ambient air for the blower to blow the heated air into the hopper so that moisture contents at surfaces of the material granules and in the hopper can be evaporated due to a high heat provided by the heated air.
However, such assembly of the hopper, the heater and the blower is bulky. On the other hand, in the conventional drying apparatus, procedures of material filling, heating drying and material discharging are performed at an identical position. Stating differently, not until a batch of material granules is processed and discharged, a next batch of material granules can not be loaded for a next cycle of material filling, heating drying and material discharging. Therefore, an overall drying process is time-consuming and, consequently, an efficiency of the injection-molding device is inferior.
Moreover, since the drying process where the heated air is used to evaporate the moisture contents at the surfaces of the material granules causes the surfaces of the material granules heated, in order to avoid condensation caused by a drop in temperature at the surfaces of the material granules when the heated material granules are transferred in to a molding machine, a timing for transferring the material granules has to be well controlled.
The U.S. Pat. No. 6,154,980 provides a dryer comprising a plurality of hoppers. The plural hoppers are rotatable to circulate among positions so as to receive different drying procedures at the different positions. For example, when one said hopper is at a material filling and heating position, a material filling and heating procedure is performed to evaporate moisture contents in the material granules. When the hopper shifts to a vacuum drying position, a vacuum drying procedure is conducted to the hopper so as to vacuum the hopper and thereby evacuate moisture contents therefrom. When the hopper is at a material discharge position, the material granules in the hopper are discharged. While the prior art device teaches a circulative drying process, the heating procedure is the outstandingly time-consuming part among all the process. Generally speaking, a heating temperature up to 150° C. and a heating duration of 25 to 30 minutes are required. After positioned at the material filling and heating position for 30 minutes, the hopper can be moved to the vacuum drying position and then the drying procedure can be done in 3 to 5 minutes. At this time, however, since the heating procedure for another said hopper at the material filling and heating position has not been finished yet, it needs 25 minutes more to get the hoppers moved again. Thus, it takes about one hour for each said hopper to undergo all the procedures from material filling through material discharge and the material discharge can be only performed every 30 minutes. Consequently, to the entire process, the heating drying procedure is either time-consuming or unsatisfying in its drying effect.